The invention relates to a device for forming an X-ray or gamma ray beam of small cross-section and variable direction, comprising an X-ray source or gamma source which supplies a radiation beam and a diaphragm device which forms the X-ray beam from the radiation beam and which comprises a stationary diaphragm section provided with a rectilinear slit and a cylindrical first diaphragm body which rotates about an axis of rotation and which is provided with a helical slit on its outer surface.
Such devices are known from EP-OS No. 74 021 for medical applications and from, which corresponds to U.S. Pat. No. 4,750,196, for industrial applications. Therein, the diaphragm body is formed by a hollow cylinder of a radiation absorbing material, the circumference of which is provided with two mutually offset helically extending slits. When a beam of parallel rays is incident on such a diaphragm body in the direction perpendicular to the cylinder axis thereof, there will always be one point in which the X-ray beam passes through both slits. When the diaphragm body is rotated, this point travels along the axis so that a periodically moving X-ray beam emerges behind the diaphragm body. This periodically displaced X-ray beam can be used for medical or industrial examinations. The stationary diaphragm section serves to bound the X-rays perpendicularly to the direction of displacement of the X-ray beam in a defined manner.
In practice X-rays are produced by means of an X-ray tube which delivers a beam of diverging rays. In a diverging radiation beam, however, the intensity of the X-ray beam decreases towards the edges. This can be at least partly prevented by suitably shaping the slits and by ensuring that instead of extending exactly parallel to the plane formed by the rectilinear slit and the radiation source, the axis of rotation of the diaphragm body encloses an angle with respect thereto which angle depends on the divergence of the radiation beam, i.e. on the distance between the radiation source and the axis of rotation of the diaphragm body.
Therefore, such a diaphragm body can be manufactured only at considerable expenditure and is suitable only for one given distance between the radiation source and the axis of rotation.